The term, “dietary fiber”, inherently means cell components of plants, which is hardly digestible by animals, such as cellulose, lignin, hemicellulose, pectin, etc., however, in the broad sense, it includes low-digestible water-soluble polysaccharides which are not digested by amylases. Such water-soluble polysaccharides are called as “water-soluble dietary fiber” (hereinafter, simply abbreviated as “WSDF”, in this specification). Recently, dietary fiber attracts attention to its functions as prebiotics improving bacterial flora in the intestine in addition to its inherent functions of regulating the functions of the intestine, lowering blood-cholesterol level, and controlling blood-sugar level. However, it is generally recognized that dietary fiber and calcium are nutritional elements which are insufficiently taken in Japanese dietary life. It has been pointed out that, in the present-day, the average intake of WSDF in Japanese is only 50 to 80% of the objective intake, 20 to 25 g/day, recommended in “Nutritional Requirement in Japanese”, 5th edition (1994) (Ref. “Market Trend of Dietary Fiber”, Shokuhin-To-Kaihatsu (Food processing and ingredients), Vol. 34, No. 2, pp. 24-27 (1999) (in Japanese)). Under these circumstances, various low-digestible polysaccharides, which can be used as materials for various foods and beverages and useful as WSDF, have been proposed.
For example, polysaccharides present in nature or their modified products, such as low-digestible starch (moist heat treated high-amylose corn starch), guar gum hydrolyzate, glucomannnan, and low-molecular weight alginate are commercially available as WSDFs. However, since they have relatively high viscosities and defects of deteriorating relish and texture when they are incorporated into foods and beverages, their uses are restricted to a narrow range. While, “POLYDEXTROSE®” (developed by Pfizer Inc., USA) and low-digestible dextrins are widely utilized in the field of foods and beverages as WSDF with low-viscosity. “POLYDEXTROSE®” is a synthetic polysaccharide obtained by the steps of heating glucose, sorbitol, and citric acid under a high-vacuum condition; and polymerizing them by the chemical reaction. It is known that “POLYDEXTROSE®” has complicated branched structures of binding glucoses via 1,3-, 1,4-, 1,6-, 1,2,6-, and 1,4,6-glucosidic linkages. On the other hand, the low-digestible dextrin is a synthetic polysaccharide whose digestibility is lowered by inducing 1,2-, 1,3-, 1,2,4-, and 1,3,4-glucosidic linkages, not inherently present in starch, formed by transglucosylation and reverse-reaction during the chemical hydrolysis of starch. The low-digestible dextrin is produced by the steps of adding a small amount of hydrochloric acid to starch, heating the mixture in a powdery form to obtain roasted dextrin, dissolving the resulting roasted dextrin into water, hydrolyzing the roasted dextrin by admixing with α-amylase, purifying the resulting solution with a low viscosity, concentrating the solution, and drying the dextrin with a spray-dryer. As a low-digestible dextrin, another product, produced by the steps of allowing glucoamylase to act on the above low-digestible dextrin to hydrolyze the digestible part into glucose, removing the resulting glucose, purifying, and drying the dextrin with spray-dryer to further lowering digestibility, has been commercialized. However, since the low-digestible dextrin can not be obtained in a high yield from material starch and it causes color-deterioration easily, these characteristics are problems on the industrial production of the low-digestible dextrin. It is reported that the newly induced glucosidic linkages in “POLYDEXTROSE®” and the low-digestible dextrin include both α- and β-anomer forms and the reducing end glucose of those are partially converted into 1,6-anhydro-glucose (Ref. “Low-molecular weight water-soluble dietary fiber”, part of a series of Science of Dietary Fibers, pp. 116-131, published by Asakura Shoten (1997)).
Among the glucosidic linkages (hereinafter, “glucosidic linkage” is simply abbreviated as “linkage” in this specification) which are a mode of binding glucose in glucan, α-1,6 linkage is less hydrolysable by amylase than α-1,4 linkage. Therefore, it is expected that glucan rich in α-1,6 linkages can be used as WSDF. For example, dextran, produced from sucrose as material by the action of dextransucrase (EC 2.4.1.5) from Leuconostoc mesenteroides belonging to lactic acid bacteria, is a glucan in which glucoses are polymerized by mainly α-1,6 linkages, and may have branches by α-1,2 and α-1,3 linkages. In the case of using dextransucrase from Leuconostoc mesenteroides B-512F, the resulting dextran has α-1,6 linkages in the ratio of 90% or higher in the linkages of the dextran, and is expected to be a low-digestible glucan. However, dextran can not be obtained in a high yield from sucrose, requires complicated purifying procedure because of its high viscosity, and drives up the cost. Therefore, dextran has not been tried to be used as WSDF.
There has been proposed a method for preparing WSDF by allowing amylase to act on inexpensive starch to hydrolyze α-1,4 linkage for relatively increasing the content of α-1,6 linkages. Japanese Patent Kokai No. 11,101/2001 disclosed a method for preparing a branched dextrin in which the ratio of α-1,6 linkage to α-1,4 linkage is increased to to 20% by the steps of allowing α-amylase and β-amylase to act on liquefied starch and collecting the residual dextrin. However, the yield of the branched dextrin from material starch is relatively low and the lowering of the digestibility can not be expected because the branched dextrin is produced by a method of increasing the ratio of α-1,6 linkage while keeping the inherent branches (α-1,6 linkages) in starch and removing glucose chain in which glucoses are polymerized via α-1,4 linkages. While, dextrin dextranase (EC 2.1.1.2) has been well known as an enzyme which acts on partial starch hydrolyzate (dextrin) and induces α-1,6 linkages in its molecule (Ref. Kazuya Yamamoto et al., Bioscience, Biotechnology, and Biochemistry, Vol. 56, pp. 169-173 (1992)). Dextrin dextranase is an enzyme which acts on partial starch hydrolyzate and forms dextran having a structure of polymerizing glucoses via α-1,6 linkages by catalyzing mainly α-1,6 glucosyl-transferring reaction. However, there are problems in the well-known dextrin dextranase from Acetobacter capsulatum belonging to acetic acid bacteria that the ratio of α-1,6 linkage inducible in the molecule is relatively low (Ref. Masayuki Suzuki et al. Journal of Applied Glycoscience, Vol. 48, No. 2, pp. 143-151 (2001)), and the enzyme is unstable. Therefore, the enzyme has not been used practically. Under these circumstances, a novel low-digestible glucan and a process for producing the same have been strongly desired for increasing options of WSDF.